1. Field of the Invention
The present invention relates to a television receiver, a channel tuning method and a channel scan method.
2. Related Art
There is known a television receiver capable of receiving television broadcast signals (hereinafter referred to as “TV broadcast signals”) of terrestrial digital broadcasting and cable television (hereinafter referred to as “CATV”) digital broadcasting (see, for example, Patent Document 1).
In such a television receiver, it is necessary to preset the channel data of a TV broadcast signal in advance by, for example, performing the auto scan of channels in the order of channels (physical channels) as default in order to rapidly receive the TV broadcast signal corresponding to the channel tuned by a user for example.
For example, a broadcasting station of terrestrial digital broadcasting in USA transmits only the TV broadcast signals modulated by a kind of a modulation method (8-level vestigial side band (VSB) modulation method). Consequently, in the case of a TV broadcast signal of the terrestrial digital broadcasting (hereinafter referred to as a “terrestrial TV broadcast signal”), channel data can be preset only by, for example, scanning channels and by judging whether the reception of a terrestrial TV broadcast signal has succeeded or not.
On the other hand, a broadcasting station of the CATV digital broadcasting in USA transmits a TV broadcast signal modulated by any one of three kinds of modulation methods (8 VSB modulation method, 64 quadrature amplitude modulation (QAM) method and 256 QAM method) using any one of three kinds of frequency tables (standard frequency (STD) table, incremental related carrier (IRC) table and harmonic related carrier (HRC) table). Consequently, in the case of a TV broadcast signal of the CATV digital broadcasting (a cable television broadcast signal (hereinafter referred to as a “CATV broadcast signal”)), channel data cannot be preset without judging whether the reception of the CATV broadcast signal has succeeded or not by severally setting, for example, nine kinds of combinations obtained by multiplying the three kinds of modulation methods by the three kinds of frequency tables, and by scanning channels to each of the setting.
That is, it takes a longer time to preset the channel data of the CATV broadcast signal in comparison with the time necessary to preset the channel data of the terrestrial TV broadcast signal.
Accordingly, various methods have been proposed in order to preset the channel data of the CATV broadcast signal in a shorter time.
To put it concretely, one proposed method sets the frequency tables in the descending order from the frequency table that is most frequently adopted in the CATV broadcasting (the order of the STD table, the IRC table and the HRC table), and judges the setting by which the CATV broadcast signal corresponding to one channel can be received. Then, the method scans the other channels in the judged setting, and judges whether the CATV broadcast signal can be received or not. Thereby, the method presets the channel data in a television receiver for the CATV digital broadcasting (see, for example, Patent Document 2). Another method presets channel data by using the fact that the probability that the combination of the frequency table and the modulation method of the CATV broadcast signal corresponding to one channel and the combination of the frequency table and the modulation method of the CATV broadcast signal corresponding to another channel are the same is high in one transmission band (one band among five bands obtained by dividing the band of the CATV digital broadcasting into the five ones) because the frequency bands of adjoining channels adjoin each other without any gaps, or by using the fact that the probability that the combination of the frequency table and the modulation method of the CATV broadcast signal corresponding to one channel in one transmission band and the combination of the frequency table and the modulation method of the CATV broadcast signal corresponding to the other channel in the other transmission band are the same is high in adjoining transmission bands because the frequency bands of adjoining channels adjoin each other without any gaps (see, for example, Patent Document 3). These and other methods have been proposed.    [Patent Document 1] JP-A-2005-94555    [Patent Document 2] JP-A-2004-363660    [Patent Document 3] JP-A-2003-230060
However, for example, the method disclosed in the Patent Document 2 cannot preset the channel data of the CATV broadcast signal corresponding to another channel unless the frequency table of the CATV broadcast signal corresponding to one channel and the frequency table of the CATV broadcast signal corresponding to the other channel are the same.
Moreover, for example, the method disclosed in the Patent Document 3 cannot preset channel data unless it is judged whether the reception of the CATV broadcast signal has succeeded or not by setting each of the nine kinds of combinations when the combinations of the frequency tables and the modulation methods are not the same in one transmission band, or when the combinations of the frequency tables and the modulation methods of adjoining transmission bands are not the same.
Moreover, the methods disclosed in the Patent Documents 2 and 3 cannot surely and rapidly perform the channel tuning of a television receiver at the time of a normal operation when channel data has been changed or the like.